1. Field of the Invention
The present invention relates to coolant flow transition pieces in bottom entry fuel assembly design reactors and, in particular, a method for modifying the design of existing transition pieces in such reactors to reduce coolant pressure drop.
2. Background Art
FIG. 1a illustrates a bottom entry fuel assembly 10 of the prior art typically used in BWR nuclear reactors. The assembly includes a number of nuclear fuel rods 11 disposed between upper and lower tie plates 12 and 13. The assembly includes at the bottom of the lower tie plate a transition section 15 for directing coolant into the fuel assembly. The transition section 15 is better shown in FIG. 1b with reference to the fuel assembly support plate 18. The transition section, hereafter referred to as the transition piece 15, may be either separate from the lower tie plate grid or it may be integral with the grid, dependent on the designed interface with the fuel channel.
Referring to FIG. 1b, the transition piece 15 between the fuel assembly and the fuel supporting plate 18 in such bottom entry nuclear fuel assemblies is subject to certain constraints. The top portion 16 must. interface with a channel in a prescribed manner and is generally square in shape. The portion 17 of the transition piece extending downward to the fuel assembly support plate 18 in FIG. 1b is generally round and must seat in a specific diameter opening of specific depth. The distance between the fuel assembly support plate 18 and the fuel assembly itself is defined. Related dimensions of one prior art design are also indicated on FIG. 1b. In this figure, A indicates the portion of the transition piece going from round to square.
In the prior design as shown in FIGS. 1a and 1b, the lower end of the transition piece 15 comprises a three-prong lead in finger 19 which is located just above the orifice 20 when the transition piece 15 is seated on the fuel assembly support plate 18. This prior arrangement allowed uncontrolled expansion of the coolant flow which resulted in a time varying jet of fluid from the orifice. This in turn contributed to fuel rod and water rod vibration which resulted in failure of lower end caps.
In this prior design, in particular, as the flow field passes through the opening in the fuel support plate, it narrows in diameter and then expands. The primary portion of the flow field remains at a velocity typical of the orifice cross section and requires several orifice diameters of downstream motion before the boundaries of the jet expand to contact the inner wall in the known standard transition piece. Because the jet does not expand to contact with the transition piece, it oscillates from side to side within the transition piece. With higher flow rates, the length of the identifiable flow field from the orifice becomes longer, i.e., more down stream diameters of flow are required to achieve contact with the standard transition piece. The oscillatory behavior of the flow field occurs over the entire length of the transition piece for flows typical of the central orifice regions of bottom entry fuel assembly type reactors.
Experiments have been performed to observe the flow passing through an orifice and standard transition piece. These experiments confirm the oscillatory behavior of the jet entering the transition piece, i.e., the jet fails to re-attach to the transition piece and fluctuates from side to side. The retention of an identifiable oscillatory flow field in the transition piece is a factor contributing to mechanical wear on the end caps of the water rods and fuel rods which seat on the lower tie plate immediately downstream of the end of the transition piece. Experiments confirm the mechanical wear on end caps in the presence of the oscillatory jet. In-reactor wear on the end caps has been observed and water rods have been observed to become disengaged from the lower tie plate due to the wear on the end caps.
While various shapes of transition pieces are known in the prior art, none have addressed the particular problem in bottom entry fuel assembly reactor designs as discussed above.